Superfinishing, also known as micromachining, microfinishing, and short-stroke honing, is a metalworking process that improves surface finish and workpiece geometry. This is achieved by removing just the thin amorphous surface layer left by the last process with an abrasive stone or tape; this layer is usually about 1 μm in magnitude. Superfinishing, unlike polishing which produces a mirror finish, creates a cross-hatch pattern on the workpiece.
After a metal piece is ground to an initial finish, it is superfinished with a finer grit solid abrasive. The abrasive is oscillated or rotated while the workpiece is rotated in the opposite direction; these motions are what causes the cross-hatching. The geometry of the abrasive depends on the geometry of the workpiece surface; a stone (rectangular shape) is for cylindrical surfaces and cups and wheels are used for flat and spherical surfaces. A lubricant is used to minimize heat production, which can alter the metallurgical properties, and to carry away the swarf; kerosene is a common lubricant.
The abrasive cuts the surface of the workpiece in three phases. The first phase is when the abrasive first contacts the workpiece surface: dull grains of the abrasive fracture and fall away leaving a new sharp cutting surface. In the second phase the abrasive "self dresses" while most of the stock is being removed. Finally, the abrasive grains become dull as they work which improves the surface geometry.
The average rotational speed of abrasive wheel and/or workpiece is 1 to 15 surface m/min, with 6 to 14 m/min preferred; this is much slower compared to grinding speeds around 1800 to 3500 m/min. The pressure applied to the abrasive is very light, usually between 0.02 to 0.07 MPa (3 to 10 psi), but can be as high as 2.06 MPa (299 psi). Honing is usually 3.4 to 6.9 MPa (490 to 1,000 psi) and grinding is between 13.7 to 137.3 MPa (1,990 to 19,910 psi). When a stone is used it is oscillated at 200 to 1000 cycles with an amplitude of 1 to 5 mm (0.039 to 0.197 in).
There are three types superfinishing: Through-feed, plunge, and wheels.
- This type of superfinishing is used for cylindrical workpieces. The workpiece is rotated between two drive rollers, which also move the machine as well. Four to eight progressively finer abrasive stones are used to superfinish the workpiece. The stones contact the workpiece at a 90° angle and are oscillated axially. Examples of parts that would be produced by process include tapered rolls, piston pins, shock absorber rods, shafts, and needles.
- This type is used to finish irregularly shaped surfaces. The workpiece is rotated while the abrasive plunges onto the desired surface.
- Abrasive cups or wheels are used to superfinish flat and spherical surfaces. The wheel and workpiece are rotated in opposite directions, which creates the cross-hatching. If the two are parallel then the result if a flat finish, but if the wheel is tilted slightly a convex or concave surface will form.
Aluminum oxide is used for "roughing" operations. Silicon carbide, which is harder than aluminum oxide, is used for "finishing" operations. CBN and diamond are not as commonly used, but find use with specialized materials such as ceramics and M50 tool steel. Note that graphite may be mixed with other abrasives to add lubricity and to enhance the appearance of the finish.
Abrasive grains must be very fine to be used with superfinishing; usually 5–8 μm.
Advantages and disadvantages
The main disadvantage is that superfinishing requires grinding or a hard turning operation beforehand, which increases cost. Superfinishing has a lower cutting efficiency because of smaller chips and lower material removal rate. Superfinishing stones are softer and wear more quickly, however they do not need to be dressed.
Common applications include: steering rack components, transmission components, fuel injector components, camshaft lobes, hydraulic cylinder rods, bearing races, needle rollers, and sharpening stones and wheels.
It has been proven that superfinishing certain parts makes them more durable. For example if the teeth in a gear are superfinished they will last up to four times as long.
- Darmann Abrasive Products, The Art of Superfinishing (PDF), retrieved 2017-02-01.
- Todd, Allen & Alting 1994, pp. 135–136.
- Murty 1996, p. 187.
- Schwarz, Jeff; Darmann Abrasive Products (1998-12-15), "For Superfinishing Excellence, Start With The Right Finish", Modern Machine Shop.
- Swigert Jr., Arthur M. (1940), The story of superfinish, Ann Arbor, MI: The Ann Arbor Press, pp. 575–594, OCLC 568009
- Krantz, Timothy L. (1 March 2000). Gear Durability Shown To Be Improved by Superfinishing (Technical report). NASA.